[petsc-users] FieldSplit, multigrid and blocksize

[Karin&NiKo]

Thank you all.
These are the answers I was looking for!

Best regards,
Nicolas

2016-12-07 15:08 GMT+01:00 Mark Adams <mfadams at lbl.gov>:

> Note, for best performance with ML and GAMG you want to give it the near
> kernel for the 00 block. These are the 6 "rigid body modes" or zero energy
> modes. PETSc provides some tools to do that (eg,
> MatNullSpaceCreateRigidBody).
>
> On Wed, Dec 7, 2016 at 4:45 PM, Lawrence Mitchell <
> lawrence.mitchell at imperial.ac.uk> wrote:
>
>>
>>
>> On 07/12/16 13:43, Karin&NiKo wrote:
>> > Thanks Barry.
>> > I must emphasize that my unknowns are not numbered in a regular way :
>> > I am using a P2-P1 finite element and the middle nodes do not carry a
>> > pressure DOF. So the global numbering is somewhat like :
>> > ------------------------------------------------------------
>> -----------------------------------------------
>> > u1x, u1y, u1z, p, u2x, u2y, u2z, p2, u3x, u3y, u3z, u4x, u4y, u4z, p4,
>> > .....
>> >  node 1 DOF     |  node 2 DOF       | node 3 DOF  |    node 4 DOF     |
>> > ------------------------------------------------------------
>> -----------------------------------------------
>> >
>> > So my global matrix does not have a block-size of 4. Nevertheless the
>> > A00 matrix has a block size of 3!
>> > Is there a way to specify that only on the A00 sub-matrix?
>>
>> I presume you are defining the splits by providing ISes.  You need to
>> set the block size on the IS that defines the A00 block appropriately,
>> then the submatrix will have it.
>>
>> Lawrence
>>
>>
>
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